Diffusion flame (nozzle mixing) gas burners are used in various industrial kilns and furnaces. There are various types of units used in such furnaces, including but not limited to self-recuperative single-ended radiant tube burners. These burners have flame contained within the tube, which tube then radiates heat into the furnace. The units are designed so that no products of combustion enter the work processing chamber of the furnace. Substantially all of the products of combustion are exhausted rearward through the burner outside of the work processing chamber of the furnace and in a direction substantially opposite the flame direction. These exhaust gases preheat the incoming combustion air by extracting waste heat from the hot exhaust gases.
Diffusion flame burners such as self-recuperative single-ended radiant tube burners must have excellent flame stabilization. If the flame front is not anchored at a fixed start location, undesirable acoustics will develop in the tube. Self-recuperative burners additionally have an effective built-in heat exchanger to transfer waste heat from the exhaust gas to the incoming combustion air. These burners typically have an axial design comprised of a flame holder and an inner flame tube. The flame holder provides fuel through a central fuel tube. Primary air to support combustion of the fuel is provided at a low velocity along a first passageway coaxial with the central fuel tube. The primary air distributes fuel as the fuel exits the central fuel tube where it is spark-ignited. Secondary air is provided at greater volumes along a second passageway coaxially with the first passageway to mix with uncombusted fuel in a combustor to provide a flame. The secondary air is separated from the primary air by a tube whose tube wall forms a boundary between the primary air and the secondary air. The secondary air may be provided at higher pressure or at higher velocities or both. As the secondary air combines with the unmixed fuel, it is projected from a reducer at high velocities into the inner flame tube, which is located axially downstream from the reducer. The mixing of the primary air and the fuel is completed in the inner flame tube which heats the industrial furnace or kiln. An exemplary self-recuperative radiant tube burner is set forth in U.S. Pat. No. 4,705,022 to Collier.
Another type of burner assembly is a radiant tube burner with two legs, and a semi-toroidal linking section, commonly referred to as “U-tubes” because the tube is in the shape of an elongated “U.” The traditional method of U-tube design is to place the burner at one end of the tube and the recuperator at the other end of the tube. The usual result from this design is that the firing leg of the tube operates at a higher temperature than the exhaust leg of the tube as the energy of the flame is dissipated down the firing leg of the tube.
Various design changes have been introduced into burner designs to improve the combustion of the burners or to reduce NOx emissions from the burners. One such design improvement is set forth in U.S. Pat. No. 5,700,143 ('143 patent) to Irwin et al. and assigned to the assignee of the present invention. The complex tube design set forth in the '143 patent swirls secondary air by introducing it into the secondary tube through spin vanes where a spin is imparted. The swirling air exits the secondary tube at the end of the primary air nozzle or slightly upstream of the primary air nozzle. The swirling secondary air assists in atomizing the fuel from the fuel/air mixture.
While the burner set forth in the '143 patent is particularly effective in permitting a change from one fuel to another, it is complex and expensive, but can introduce undesirable acoustics.
A difficulty that can be encountered by burners that rely on swirling air to improve mixing is that the secondary air is introduced and swirled at a first end, but exits the secondary tube at a second end. Under steady state conditions, the swirl imparted to the air appears to be stable. The swirling air modifies the acoustics of the burner, producing undesirable acoustics that are very unpleasant, and potentially damaging noise, to anyone in the vicinity. Additionally, with changing conditions as the air flow is modified, the effects within the tube can change the nature of the swirl, causing the flame to be unstable. In certain extreme situations, this can impact the combustion process, such as by extinguishing the flame.
Another problem associated with burners is efficiency. The efficiency of the burners can be improved as the temperature of the secondary air is increased. In radiant burners such as the one described in the '143 patent as well as other burners, the secondary air is frequently used to cool the metallic parts that comprise the burner, as the elevated temperatures of combustion can destroy these parts, if adequate cooling is not provided. Of course, the secondary air is also heated, but this effect is limited by the heat transfer characteristics of the assembly.
Another problem that impacts upon burners, as noted above, is flame stabilization. If the flow of secondary air is altered, either by increasing or decreasing air flow, it is possible to move the flame front and/or extinguish the flame that exists at the juncture of the primary air tube and fuel tube. Thus care must taken when adjusting air flow so as not to extinguish the flame.
What is needed is a diffusion flame burner that can provide a swirling component to secondary combustion air in a manner to stabilize the flame over a broad range transient conditions, and that does not produce undesirable acoustic effects. The burner should also allow changes in secondary air flow without destabilizing the flame. It should also heat the secondary combustion air to improve the efficiency of the combustion process. The burner should also be able to be used with retrofitted U-tube radiant burners or newly installed U-tube radiant burners. The diffusion flame burner of the present invention should be a simple design to construct, and should be resistant to damage resulting from long exposures to heat, high temperature and flame.